Leave the salary up for discussion during the interview. If they ask for it, I tell them outright what I was making. Pretty rare for a company to ask though these days.
Going further, I would be very very leary about putting any form of salary expectations in a cover letter. With the economic/financial instability, wages/salaries are now changing and demanding $50K one week then having others willing to do the same work for less the following weeks is just asking for your letter to be thrown into the rectangular filing cabinet sitting on the floor.
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